In an MRI system or MR scanner, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei, namely their gyromagnetic ratio, and the strength of the applied B0 field. The gyromagnetic ratio is the ratio between the magnetic moment and the spin of a nucleus.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. NMR relaxation signals which are emitted by the transversal relaxation process, are detected by means of an RF receive antenna. The received NMR signals which are time-based amplitude signals, are Fourier transformed to frequency-based NMR spectrum signals and processed in a known manner for generating an MR image of the examination object.
The above RF transmit and/or receive antennas are known both in the form of so-called MR body coils which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called MR surface coils which are directly arranged on a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages (head coil or birdcage coil). For examining a local zone or area, interventional devices like catheters or another invasive device comprising an RF transmit and/or receive antenna especially in the form of a small coil or loop for receiving MR relaxation signals are also known.
As to the shape of the examination space, two types of MRI systems or MR scanners can be distinguished. The first one is the so-called open MRI system (vertical system) which comprises an examination zone, which is located between the ends of a vertical C-arm arrangement. The second one is a horizontal MRI system, also called axial MRI system, which comprises a horizontally extending tubular or cylindrical examination space.
U.S. Pat. No. 5,457,387 discloses an axial MRI apparatus in which an RF coil assembly is provided having a pair of end rings supported by a gradient coil assembly, wherein between the pair of end rings a plurality of longitudinally coil elements extend which are mechanically and selectively attachable and detachable to/from the gradient coil assembly. By such an RF coil assembly, critical diameter reductions within the magnet bore of the MRI apparatus shall be prevented and a larger patient aperture shall be obtained.